1. Field of the invention
The invention concerns electrostatic sprayer apparatus for spraying material such as paint, and more particularly an improvement for avoiding electrical arcing which can represent a real danger when the sprayed material contains volatile, inflammable substances.
2. Description of the prior art
Electrostatic sprayer apparatus comprises in particular a mobile sprayer equipped with a high voltage electrode, designed to create an electrical field between said electrode and the object to be sprayed (which is grounded) and to disperse the sprayed material. To achieve this, the sprayer apparatus comprises, in addition to the electrode, means of supplying the sprayed material with spraying energy (air) and electrical energy. The sprayer is therefore connected to a fixed supply and control unit by a supply line comprising electrical cables and other conduits. Electrical energy is almost always output by the sprayer in the form of a high DC voltage on the order of 50 to 150 kV, applied to the electrode. The resulting current, represented by the flow of charged particles and the atmospheric ions generated or attracted by the field, is on the order of 50 to 200 .mu.A. A number of methods has been proposed for applying the high voltage to the sprayer electrode.
The high voltage may be applied directly to the electrode from a high voltage generator in the remotely sited supply and control unit, which therefore contains the converters for converting the electrical energy supplied by the AC distribution network into a high DC voltage. The supply line must then include a high voltage cable in an appropriately adapted insulating sheath.
The sprayer may also be supplied from a very low DC voltage (a few tens of volts) if the sprayer includes a cascade arrangement of a high frequency oscillator supplied from said very low DC voltage, a step-up transformer outputting a high frequency medium voltage (on the order of a few kV) and a conventional capacitor/diode voltage rectifier-multiplier assembly transforming the high frequency medium voltage into a high DC voltage, which is then applied to the electrode. In this case, the supply and control unit contains only a step-down transformer and a rectifier to transform the low AC voltage of the distribution network into a very low DC voltage. The said supply line includes conductors (without strengthened insulation) carrying this direct current to the oscillator.
Another proposed solution is to locate only the transformer and voltage rectifier-multiplier assembly in the sprayer. The oscillator is then remotely located in the supply and control unit. In this case, the supply line carries a very low voltage, high frequency AC signal. The invention to be described applies preferably to such an arrangement although its principle may be applied to any system comprising DC/high frequency AC conversion.
Electrical arcing observed in operation may result from a number of causes.
Whatever device is used, it may arise that the distance between the high voltage electrode of the sprayer and the grounded object to be sprayed becomes insufficient to support the value of the high DC voltage.
If the high voltage is generated at a location remote from the sprayer, an insulation fault in the high voltage cable may also be a cause of electrical arcing.
However, if the high voltage is generated in the sprayer, the supply line carries a relatively high current (a few amps) and an interruption in the cable or a poor connection contact is likely to cause an electrical arc because of the energy stored in the inductance of the circuit.
Various systems have been proposed to combat electrical arcing. These known systems are classified according to the type of electrical arc that it is wished to eliminate.
Thus, it is known that arcing can be prevented by monitoring the DC current of the voltage rectifier-multiplier assembly, generally at the point of its return to the power supply. Electrical arcing involves a sudden increase in current in this cirucit and this increase in current may be used by electronic means to act on the high voltage generation means. Such a device is described for example in French Pat. No. 2397735. The disadvantage of many devices of this type is that any failure in the monitoring means (in series with the high voltage supply) generates interference likely to cause the destruction of certain components of the power supply. If the ground connection of the voltage rectifier-multiplier is interrupted, a substantial increase in voltage occurs on the corresponding terminal of the high voltage supply, this increase in voltage being dangerous for the components. In addition, the return current is often used as a negative feedback signal to regulate the power supply, a reduction in current being used in a control device to implement an increase in the high voltage. Consequently, a failure in the monitoring means may bring about an undesirable increase in the high voltage. These risks are substantially greater in devices in which the high voltage generation means are incorporated in the sprayer since the current return conductor passes into the supply line and is therefore more vulnerable by virtue of its length. Also, the monitored current is not strictly speaking direct, but rather a "unidirectional ripple" current and it includes an AC component linked to the spurious capacitances of the voltage multiplier, this AC component being superimposed on the DC component. Consequently, any means of detecting the increase in this current must include effective filtering to avoid the edge of the AC component being "interpreted" by the monitoring circuits as an increase in current. The need for such filtering often involves poor response time of the monitoring circuit, which is therefore always very complex and consequently expensive and fragile.
It has consequently been proposed to control the rapid changes in high voltage which precede electrical arcing by using a capacitive divider. This principle is described in French Pat. No. 1601577. This solution is particularly effective when arcing is more particularly due to a reduction in the distance between the high voltage electrode and the object to be sprayed.
Finally, a method is known of monitoring the low voltage direct current when the sprayer is supplied by this low voltage (that is when it contains the oscillator and rectifier-multiplier) by comparing this current with a reference value. Such a device is for example described in French Pat. No. 2 551 928. The only function of the monitoring means is to monitor the continuity of the circuit in order to react to a sudden fall in the oscillator supply current. The monitoring means can therefore detect only this type of fault and do not eliminate the need to use a monitoring circuit for the high voltage itself, similar to one of those described above, with the aforementioned disadvantages.